Series 3000 Layout Guidelines

Transcription

1 Included are the design layout guidelines for Series 3000 Cooling Towers in several situations typically encountered by designers. These guidelines represent minimum spacing requirements. If available, greater spacing should be utilized whenever possible. Overview Operational efficiency of evaporative cooling equipment depends upon an adequate supply of fresh, ambient air to provide design capacity. Other important considerations, such as the proximity to building air intakes or discharges, also must be taken into account when selecting and designing the equipment site. As the size of an installation increases, the total amount of heat being rejected into the atmosphere and the volume of discharge air increases to the point where the units can virtually create their own environment. As a result, it becomes increasingly difficult to apply a set of general guidelines to each case. In such installations, particularly those in wells or enclosures, some air will recirculate. The recirculation should be minimized or design wet-bulb temperature must be adjusted to allow for the recirculation. Consequently, any job that involves four or more cells should be referred to your local BAC Representative for review. Did you know? As the size of an installation increases, the total amount of heat being rejected into the atmosphere and the volume of discharge air increases to the point where the units can virtually create their own environment. Axial fan equipment units are not generally suited for indoor or ducted applications. In such situations, a Series V centrifugal fan unit is recommended. 1

2 General Considerations When selecting the site consider the following factors: 2 from casing 4 from connection Figure 1: Plan View of the Recommended Unit Servicing and Maintenance Spacing for a Dual Unit 1. Locate the unit to prevent the warm discharge air from being introduced into the fresh air intakes of the unit s building(s), intakes of neighboring buildings, or from being carried over any populated area such as a building entrance. 2. Consider the potential for plume formation and its effect on the surroundings, such as large windowed areas, and pedestrian or vehicular traffic arteries, particularly if the unit(s) will be operated during low ambient temperatures. 3. Provide sufficient unobstructed space around the unit(s) to ensure an adequate supply of fresh, ambient air to the air intake. Avoid situations that promote recirculation of unit discharge air, such as units located: a. Adjacent to walls or structures that might deflect some of the discharge airstream back into the air intake. b. Where building air intakes or exhausts, such as boiler stacks in the vicinity of the unit, might raise the inlet wet-bulb temperature or starve the unit of air. 4. Provide adequate space around the unit for piping and proper servicing and maintenance, as shown in Figure The top of the fan discharge cylinder, velocity recovery stack, or discharge sound attenuation must be at least level with, and preferably higher than, any adjacent walls or buildings. 6. When possible, orient the unit so the prevailing summer wind blows the discharge air away from the air intakes of the unit(s). 7. When the unit is installed with intake sound attenuation, the distances given in the tables below should be measured from the face of the intake sound attenuation. 8. On larger unit installations, the problem of ensuring an adequate supply of fresh, ambient air to the tower intakes becomes increasingly difficult. See the Multicell Installations Section of this article for specific considerations. 2

3 9. If the installation does not meet the recommended guidelines, the units will have a greater tendency to recirculate, and the design conditions should be altered to include an allowance for the recirculation. For instance, if the design conditions are 95 F/85 F/78 F and it was estimated that the allowance for recirculation rate was 1 F, then the new design conditions would be 95 F/85 F/79 F, and the units should be reselected based on the new design conditions. The Layout Guidelines describe several typical site layouts for BAC s cooling towers, closed circuit cooling towers, and evaporative condensers. If these guidelines do not cover a particular situation or if the layout criteria cannot be met, please refer the application to your BAC Representative for review. Please indicate prevailing wind direction, geographic orientation of the unit(s), and other factors such as large buildings and other obstructions that may influence layout decisions. Installations Adjacent to a Building or Wall(s) 1. Unit Orientation: When a unit is located near a building wall, the preferred arrangement is to have the unit situated with the cased end or blank-off side (unlouvered side) facing the adjacent wall or building. 2. Air Inlet Requirements: Should it be necessary to install a unit with the air intake facing a wall, provide at least distance d between the air intake and the wall, as illustrated in Figures 2a and 2b. 2 Envelope Area 2 d H h d Air Intake Envelope Area Figure 2a. Plan View of a Unit Adjacent to Wall Figure 2b. Section View of a Unit Adjacent to Wall 3

4 Below is the method for determining the minimum acceptable dimension d for a unit located with the air intake facing a solid wall: The maximum acceptable envelope air velocity is 300 FPM, as illustrated in the following equation: Envelope Velocity = Airflow / Envelope Area < 300 FPM The envelope area as illustrated on Figures 2a and 2b on Page 3 is [(L +2 +2) * d) + 2(H+h) * d)], where: H height of the air intake face in feet h elevation of the unit from the roof/ground/pad in feet. The maximum elevation is four feet. length of the air intake in feet Note: The maximum acceptable envelope air velocity is 300 FPM. d minimum acceptable distance between the wall and the air intake face in feet Therefore, d = Unit Airflow * 0.70 Velocity [L + 2 (H+h) + 4] The minimum acceptable dimension d for the products is tabulated in Tables 1 on Page 5. The distance d was calculated using the largest horsepower model in the box size. Example: Model S3E S Adjacent to a Solid Wall What is the minimum distance required between the air intake of the S3E S when installed facing a wall? Unit Airflow = 262,850 CFM * 3 cells = 788,550 CFM Note: Series 3000 units are dual air intake units, therefore the air intake airflow is half of the total unit airflow. Airflow = Unit Airflow = 788, Airflow = 394,275 CFM H = 19 9 = 19.75, h = 0 Solving for d, d = Airflow 300 [L+4+2(H+h)] d = 394, [ ( )] d = L = 42 3 = This is rounded to the next 0.5 increment. Therefore, the air intake should be located no less than 15.5 from the solid wall. 4

5 One Cell Two Cell Three Cell Four Cell Model Number h=0 h=2 h=4 h=0 h=2 h=4 h=0 h=2 h=4 h=0 h=2 h=4 S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x Table 1. Minimum Acceptable Distance d (feet) to Solid Wall for Series 3000 Cooling Towers for One and Two Cells Note: Refer to Figure 2b on Page 3 for location of d and h. 5

6 Well Layout The following method is used to determine the minimum acceptable dimension d for units installed in a well layout. The maximum allowable downward air velocity for a well installation is 400 fpm. The downward velocity is determined using the following equation: Downward Air Velocity = Airflow / Usable Well Area < 400 fpm The usable well area at each air intake face is defined as illustrated in Figure 3. Usable Well Area = 2 * [(d)(l+4 +4 )]+[(4 * 1 ) + (4 * 1 )], where Note: The maximum allowable downward air velocity for a well installation is 400 fpm. d minimum acceptable distance between the air intake of the unit and the wall of the well in feet length of the air intake of the unit in feet Therefore, [ ] d = (Unit Airflow *0.50) - 8 * (L+8) The minimum acceptable distance d for well installations is tabulated in Table 2 on Page 5. Usable Well Area d Air Intake d Air Intake Air Intake Air Intake 4 (typ) 1 Figure 3. Plan View of Dual Unit in a Well Enclosure 6

7 Model Number One Cell Two Cell Three Cell Four Cell S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x S3E/XES3E x Table 2. Minimum Acceptable Distance d (feet) for Series 3000 in a Well Enclosure 7

8 Louvered or Slotted Wall Installations Check to see if the layout meets the requirements for a well installation. If the criteria for the well installation are met, the layout is satisfactory. If the layout does not satisfy the criteria for the well installation, analyze the layout as follows: Louvered Wall d Max 3 Min 3 Min d Max Figure 4. Plan View of Dual Unit in Enclosure with Louvered Walls Louvered Wall 1. Air intake requirements: a. Units should be arranged within the enclosure such that the air intake directly faces the louver or slot locations as shown in Figure 4, with a minimum distance of three feet. b. If the available space does not permit the unit to be arranged with the air intakes facing the louvered or slotted walls and the enclosure cannot be modified to permit such an arrangement, consider the alternative illustrated in Figure 5 on Page 9. This arrangement should be restricted to one-cell or two-cell installations. The effective area of the louvers is only the length extending beyond the width of the unit. 2. Louver requirements: a. Louvers must provide at least 50% net free area to ensure that the unit airflow is not reduced due to friction or dynamic losses and that sufficient air is drawn through the openings and not downward from above. b. The required total louver or slot area is based on drawing the airflow through the net free area of the louvers at a velocity of 600 FPM or less. c. Locate the louver area in the walls of the enclosure such that air flows uniformly to the air intakes. d. If the unit is elevated to ensure the discharge is at the same level or above the top of the enclosure, it is acceptable to extend the louvered or slot area below the base of the units up to two feet if needed to achieve the minimum gross louver area. To calculate air velocity through the louver, the usable louvered or slot area may extend beyond the ends of the unit, by two feet maximum on either side. 8

9 Alternative Louvered Wall Area (Typ. of 4 sides) Figure 5. Plan View of Dual Unit with Alternate Louver Arrangement Calculate the louver velocity as follows: Louver Velocity = Airflow (CFM) < 600 fpm % Louver Free Area * Usable Louver Area (sq ft) The Layout Guidelines describe several typical site situations involving evaporative cooling products. If these guidelines do not cover a particular situation or if the layout criteria cannot be met, please refer the application to the your local BAC Representative for review. Please indicate prevailing wind direction, geographic orientation of the unit(s), and other factors such as large buildings and other obstructions that may influence layout decisions. 9

10 Example: S3E M-2 in a Louvered Enclosure The enclosure is 27.5 long by 38 wide by 10 tall. The enclosure walls are equal in elevation to the unit discharge height. The louvers are 70% free area and 3 0 from the air intake of the tower. The louvers extend the full width of the enclosure (38 ) on both air intake ends and they extend 9 vertically of the 10 enclosure height. Unit Airflow = 120,200 CFM x 2 cells = 240,400 CFM Note: Series 3000 units are dual air intake units, therefore the air intake airflow is half of the total unit airflow. Airflow = Total Unit Airflow/2 = 240,400/2 = 120,200 CFM L = 23' 10" = 23.83', L/2 = 23.83'/2 = 11.9' L/2 > 6', therefore e = 6' Maximum Useable Louver Length = L + 2e = (2*6) = 35.83' (of total 38 louver length) Area = 35.83*9 = ft 2 Louver Velocity = Airflow (% Louver Free Area) * (Useable Louver Area) = 120, * = 527 FPM Therefore, louver sizing is sufficient because 532 FPM < 600 FPM maximum allowable louver velocity. 10

11 Multi-Cell Installation Multiple cells create a wall of moist discharge air which could easily be swept into the air intakes due to prevailing wind. To minimize the potential of recirculation of the discharge air, the units should be situated with adequate spacing between air intakes. When multiple cells are arranged with the air intakes facing each other, the distance between air intakes should follow the equation below: M Figure 6: Plan View of Multi-cell Units with s Facing Each Other M = (2 x d) + (number of cells per module), where d is obtained from the appropriate model in Wall Layout section, Table 1 on Page 5. 11

12 Example: Model S3E R (see Figure 14) There are two banks of three cell unit modules on a roof. There are no enclosures surrounding the unit installation. The two banks of units have air intakes facing each other. What is the minimum distance M between banks of units? From Table 1 on page 7, d = 15 M = (2 * d) + (number of units per module) = (2 * 15) + 3 = 33 feet The calculated M dimension of 33 feet will minimize the potential for recirculation of the discharge air. Group the units in two cell or three cell groups, spaced at least one unit length between adjacent end walls to allow fresh air to circulate around each group, as shown in Figure 7. Figure 7: Plan View of Recommended Multi-cell Installation In the extreme case, when multiple cells are arranged with the air inlets facing each other in a well, the dimensions between the cells should follow the following requirements. 1. Enclosure: The bottom 3 feet of the well should be louvered a minimum of 50% net free area to allow air flow under the units as shown in Figure 9 on Page Support: The units should be raised off the roof deck to allow fresh air to flow under to the air inlets between the bank of cells. 3. The distance between the cells should be determined using the following method: a. Determine the maximum airflow drawn from the louvered area using the following equation: CFM drawn through the louvers = Louver Velocity x % Louver Free Area x Usable Louver Area CFM drawn through the louvers = 800 FPM x % Louver Free Area x Usable Louver Area b. Determine the CFM drawn from the top of the enclosure using the following equation: CFM drawn from the top of the enclosure = Downward Velocity x Usable Well Area 12

13 D Overall Length 4 4 D D Figure 8: Plan View of Multi-cell Installation Figure 9: Elevation View of Recommended Enclosure 13

14 Example: (10) S3E S (See Figure 8) Bottom 3' of enclosure is louvered with 50% net free area. What is the minimum distance "D" between the banks of cells. Unit Airflow = 244,030 CFM * 10 cells = 2,440,300 CFM L = 11' 10" = 11.83' Useable Length = (Number of cells * L) + (Number of Openings * L) + 4' + 4' Useable Width = 4D = (5 * 11.83) + (1 * 11.83) = 78.98' feet CFM Drawn Through Louvers = Louver Velocity * % Louver Free Area * Useable Louver Area = Louver Velocity * % Louver Free Area * Perimeter of the Enclosure * Height of Louvered Wall = Louver Velocity * % Louver Free Area * [2(Useable Length + Useable Width) * Louver Height] = 600 * 0.5 * [2( D) * 3] = 142, ,200D CFM Drawn from Top of the Enclosure = Downward Velocity * Useable Area = Downward Velocity * (Useable Length * Useable Width) = 400 * * 4D = 400 * 4D * 80 = 126,368D Total CFM = CFM Drawn through louvers + CFM Drawn from Top of the Enclosure 2,449,300 = 142, ,200D + 126,368D 133,568D = 2,298,136 D = 17.21' The Layout Guidelines describe several typical site situations involving evaporative cooling products. If these guidelines do not cover a particular situation or if the layout criteria cannot be met, please refer the application to the your local BAC Representative for review. Please indicate prevailing wind direction, geographic orientation of the unit(s), and other factors such as large buildings and other obstructions that may influence layout decisions. 14